The present invention generally relates to gas-liquid contactors and absorbers used in the removal of acidic gases, such as from utility and industrial flue gases. More particularly, this invention is directed to a wet flue gas desulfurization process and apparatus that use an ammonia-containing scrubbing solution to remove sulfur dioxide and other acidic gases from flue gases, and are further capable of reducing the presence of free ammonia in the scrubbed flue gases.
Gas-liquid contactors and absorbers (hereinafter, absorbers) are widely used to remove substances such as acidic constituents and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern is sulfur dioxide (SO2) produced by the combustion of fossil fuels and various industrial operations. Sulfur dioxide and other acidic gases are known to be hazardous to the environment, and therefore their emission into the atmosphere is regulated by clean air statutes. The method by which acidic gases are removed with a gas-liquid absorber or other type of flue gas scrubber is known as wet flue gas desulfurization (FGD).
The cleansing action produced by gas-liquid absorbers is typically derived from the passage of gas through a tower cocurrently or countercurrently to a descending liquid that absorbs the acidic gases. A conventional configuration for a gas-liquid absorber includes a tower equipped with an inlet duct through which combustion (flue) gas enter and rise through the tower. Above the inlet duct, the tower is equipped with multiple banks of spray headers or other suitable devices to introduce a contact medium, typically an alkaline slurry or solution, into the tower. Intimate contact between the contact medium and the flue gas rising through the tower results in a cleansing action in which certain gases in the flue gas are entrapped with the contact medium. The cleansed (scrubbed) flue gas continues to rise through the tower, typically passes through a mist eliminator, and may then be heated or passed directly to the atmosphere. The contact medium falls within the tower and accumulates in a tank, from which the contact medium is recycled to the spray headers. While in the tank, the absorbed gases can be reacted to produce a byproduct that is removed before the contact medium is returned to the spray headers.
Calcium-based slurries, sodium-based solutions and ammonia-based solutions are typical alkaline scrubbing liquids used in flue gas scrubbing operations. While gas-liquid absorbers utilizing calcium-based slurries generally perform satisfactorily, their operation results in the production of large quantities of wastes or gypsum, the latter having only nominal commercial value. In contrast, ammonia-based flue gas desulfurization processes have been used in the art to produce a more valuable ammonium sulfate ((NH4)2SO4) fertilizer, as taught by U.S. Pat. Nos. 4,690,807 and 5,362,458, each of which are assigned to the assignee of the present invention. In these processes, referred to as ammonium sulfate flue gas desulfurization (AS-FGD), as a flue gas flow upward through a tower, acidic gases present in the flue gas are absorbed by an ammonium sulfate solution containing ammonia. Afterwards, the solution is accumulated in a tank, where the absorbed sulfur dioxide reacts with additional ammonia injected into the tank to form ammonium sulfite ((NH4)2SO3) and ammonium bisulfite (NH4HSO3), which are oxidized with air or oxygen injected into the tank to form ammonium sulfate and ammonium bisulfate (NH4HSO4), the latter of which reacts with ammonia in the tank to form additional ammonium sulfate. A portion of the ammonium sulfate solution and/or ammonium sulfate crystals that form in the solution can be drawn off to yield the desired ammonium sulfate fertilizer byproduct of this reaction. A sufficient amount of ammonium sulfate is preferably removed from the ammonium sulfate solution prior to delivery to the tower in order to maintain ammonium sulfate at a desired concentration in the solution.
In addition to being required to react with sulfur dioxide to produce ammonium sulfate, ammonia also serves to increase the efficiency of sulfur dioxide removal by reducing the acidity of the ammonium sulfate solution introduced into the tower. With the absorption of sulfur dioxide in the tower, the ammonium sulfate solution becomes more acidic and its ability to absorb sulfur dioxide is reduced. For example, without added ammonia the pH of the ammonium sulfate solution may be in the range of about 4 to about 5.5, but with added ammonia the solution has a higher pH, for example about 5 to about 6, depending on control set points and operating conditions, including the SO2 concentration in the flue gas. However, oxidation of an ammonium sulfite solution is slower with higher pH levels. Higher pH levels are also associated with the release of free ammonia from the solution, often termed “ammonia slip.” In addition to incurring an economic loss because of lost ammonia, free ammonia in the scrubbed flue gases tends to react with any uncaptured sulfur dioxide and trioxide to create an ammonium sulfate aerosol that is visible as a blue or white plume in the stack discharge, leading to secondary pollution problems.
Controlling the amount of free ammonia in the desulfurization process is in part a function of the ammonia vapor pressure, which results from a combination of pH and levels of unoxidized ammonium sulfite that remain in the absence of sufficient oxygen. Ammonia slip can be controlled with, for example, a wet electrostatic precipitator (ESP) installed at the top of the absorber or installed in a separate module downstream of the absorber. However, a drawback is that wet ESPs are expensive and consume considerable power, making them expensive to install and operate. Another method used to control ammonia slip is to operate the scrubbing system at the lowest pH possible, coupled with a high liquid-to-gas ratio (L/G). While this method does not require additional equipment, it at best can only lower the slip to about 10 ppm, and any upsets in the control will cause higher spikes.
In view of the above, there is an ongoing effort to promote efficient oxidation rates and reduce the release of free ammonia in desulfurization processes that use ammonium sulfate scrubbing solutions.